Peripheral smear examination is the gold standard investigation in the study of hematology. It is the true window of bone marrow, the home of all hematopoietic cell production. The wedge slide (push slide) technique developed by Maxwell Wintrobe remains the standard method for the Preparation of peripheral blood smears (films). This highlights the importance of validation of numerical data retrieved by the cell counter and their interpretation. This chapter discusses various morphological abnormalities of RBCs can be Anisocytosis, poikilocytosis, hypochromic etc. and their clinical significance. This chapter also discusses the diagnosis of various parasitic infection like malaria, filaria, leishmania etc. and their diagnosis on peripheral smear.